A Roberval mechanism is widely used as a mechanism that transmits a load of a weighing object to a sensor main body in an electromagnetic balance, a strain gauge electronic balance, a capacitance balance using a capacitance change, and so on.
A Roberval mechanism has a structure in which a floating frame that receives a load of a weighing object and a fixing frame disposed opposite to the floating frame and fixed to a case or the like are connected by upper and lower sub-frames having thin portions serving as hinges near both end portions and disposed one above the other in parallel to each other, and in principle, even when eccentric loading (four-corner error) occurs on a weighing pan supported by the floating frame, a horizontal component of a moment load that is caused by the eccentric loading and applied to a sensor part is transmitted to the upper and lower sub-frames and canceled, and only a vertical component is transmitted to the sensor main body.
Thus, when a parallelism between the upper and lower sub-frames is lost, an error of the horizontal component is directly reflected in a measured value, so that, in a high-accuracy mass sensor including a Roberval mechanism, parallelism adjustment to match the heights of the upper and lower sub-frames is performed.
This height adjustment is commonly performed by cutting adjustment points (thin portions). However, this cutting method is not reversible, and an adjustment error caused by frictional heat is a concern, so that there are mass sensors provided with a reversible four-corner-adjusting-mechanism by use of a screw.
For example, the mass sensor described in Patent Literature 1 has a four-corner-adjusting-mechanism in which an upper sub-frame is fixed on cantilevered arm portions provided to extend from one end portion of a fixing frame toward an inner side of the sensor, and adjusting screws are inserted through substantially central portions of the arm portions and screwed to the fixing frame. Between the arm portions and the fixing frame, coiled springs to bias restoration of the arm portions when the adjusting screws are loosened are provided. With this four-corner-adjusting-mechanism, by rotating the adjusting screws, the upper sub-frame rises and lowers, and a parallelism between upper and lower sub-frames can be adjusted.